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I have wired up a 74HC595N with an Uno, and the standard 8 LEDs and resistors, per the usual youtube "shift register tutorial" and written a sketch that works. For example, I wrote a routine to walk from values 0 through 255 and light up the LEDs as a hex number. So my wiring and code are correct.

As an experiment, I tried disconnecting the pin on my breadboard's power rail that goes to the Arduino 5V, and the pin that goes to Arduino ground, and instead plugged in a breakout (if that's the right word) that plugs into the breadboard and provides 5V on the power rail when a 9V battery is plugged into it. (It's been checked with a multimeter, it's just under 5V.) The only pins still going to the Arduino are for DS, SHCP and STCP.

Things sort of work, but not really. It seems clear to me that the shift register keeps getting reset. (All of the LEDs briefly light back up in each iteration as I'm running my hex number display routine.) All of the other wiring remains the same; specifically OE-bar is still connected to ground on the power rail. But it still seems to reset.

Can someone explain why the shift register keeps resetting? Also, am I doing anything that might burn up the Arduino and/or shift register?

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All ground connections except those which are explicitly isolated must be connected together in order to provide a common voltage reference for the circuit. Any breaks in the ground connection can result in gross misbehavior such as what you are experiencing.

  • Thanks. Can you elaborate on what "explicitly isolated" means, or is that too broad of a question? – dnuttle Jun 12 '16 at 1:51
  • Also. Let's say that I wanted to chain shift registers together, to the point that they needed too much current for the Arduino to handle. Does that mean that this is just a brick-wall limitation of the Arduino? – dnuttle Jun 12 '16 at 1:52
  • Optocouplers and the like. No, just provide a more powerful external 5V supply for both the Arduino and the shift registers. – Ignacio Vazquez-Abrams Jun 12 '16 at 1:53

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